Incandescent lamp filaments emit visible and non-visible radiation when an electric current of sufficient magnitude is passed through the filament. A substantial amount of the energy radiated by an incandescent lamp filament, however, is in the form of non-visible radiation. As a consequence, the radiative efficiency of a typical tungsten filament, measured by the ratio of power emitted at visible wavelengths to the total radiated power over all wavelengths, is relatively low, of the order of 6.0% or less.
It has been observed that the radiative efficiency of such common filament materials as tungsten can be increased by texturing the filament surface with submicron sized features. An article entitled "Selectively Emissive Refractory Metal Surfaces," 38 Applied Physics Letters 74 (1981), by H. G. Craighead, R. E. Howard, and D. M. Tennant states that such improved radiative efficiency results from an increase in the emissivity of visible light from the tungsten. In the present context, emissivity is defined as the ratio of the radiant flux, at a given wavelength, from the surface of a substance (such as tungsten) to the radiant flux emitted under the same conditions by a black body. The hypothetical black body is assumed to absorb all and reflect no radiation incident upon it. The Craighead article, which is incorporated herein by reference, states that the emissivity of visible light from a textured tungsten surface was found to be twice that of a non-textured surface and suggests that the increase is the result of a more effective coupling of the electromagnetic radiation from the tungsten to free space. The textured surface of the tungsten sample described in the Craighead article had depressions in the surface separated by columnar structures having a cross-section of approximately 0.15 micrometers (microns) and a height above the filament surface of approximately 0.3 microns. These features were randomly arranged and were formed on the surface of the tungsten sample using a non-selective reactive ion etching technique.
A similar suggestion for enhancing incandescent lamp efficiency by modifying the surface of a tungsten lamp filament appears in a paper entitled "Where Will the Next Generation of Lamps Come From?", by John F. Waymouth and dated September 1989, and which is incorporated herein by reference. See pages 22-25 and FIG. 20. In this paper Waymouth hypothesizes that filament surface perforations measuring 0.35 microns across, 7 microns deep, and with walls 0.15 microns thick, would serve as waveguides which would effectively couple radiation in the visible wavelengths between the tungsten and free space, but inhibit the emission of non-visible radiation from the filament. As compared to a conventional filament, the radiative efficiency of such a filament would be increased and less electrical energy would be required to produce the same lamp brightness. Waymouth observes that the perforations on the filament would need to be produced by semiconductor lithographic techniques, but that the dimensions described above are beyond current state-of-the-art capabilities in this area of technology.
To achieve a high radiative efficiency, such as described by Craighead et al. or Waymouth, requires that submicron-sized surface features be formed on the lamp filament. Filaments are typically cylindrical or ribbon-shaped tungsten wires, portions of which may be coiled. The aforesaid surface features preferably cover substantially the entire filament surface intended to emit visible light. Such a requirement presents a problem, particularly where curved filament wires are concerned, because conventional semiconductor lithographic techniques for producing submicron-sized surface features are designed for use with a flat substrate, flatness being critical to creating high definition patterns. Also, unlike semiconductor fabrication, the method used to produce submicron-sized surface features on a lamp filament need not have perfect or near perfect feature yields. An adequate increase in lamp performance can be achieved with feature yields of approximately 90%. Another area of concern is that, at the normal operating temperatures of tungsten filaments, between 2000.degree. C. and 2500.degree. C., atom migration of the filament material along the surface can result in deformation or obliteration of submicron-sized surface features. Thus, it is beneficial if the surface features are fabricated in a manner that makes them resistant to such atom migration.